Met Flashcards
At what distance is visibility measured in km instead of m?
More then 5000m
Describe the composition of the atmosphere.
Nitrogen 78.09% Oxygen 20.95% Argon 00.93% Carbon Dioxide 00.03%
What are the layers of the atmosphere from highest to lowest?
Ionosphere Mesosphere Stratosphere Troposphere
What are the depths of the troposphere?
52,000ft/16km at the equator 36,000ft/11km at 50degrees north 30,000ft/9km at the poles
Describe the conditions of the stratosphere
Extends from the troposphere to 50km with a constant temp of 0 degrees Celsius
What is the difference between official and unofficial met observations?
Official is provided by a appropriately trained person at a unit with the necessary equipment. Otherwise unofficial.
What are the conditions of the ISA?
International standard atmosphere .pressure of 1013.25hPa .air density 1225gm/m3 .temperature +15degrees Celsius . 1.98 degrees per 1000ft lapse rate
What is insolation?
The incoming radiation from the sun warming the earth, as the air does not absorb the energy.
What is specific heat capacity?
The energy required to heat 1kg of a substance by 1degree Celsius.
What factors affect diurnal variation?
Surface- variation is greater over land and sea. Wind speed- wind causes turbulence that spreads the cooling effect through a greater depth of air. Cloud- reduces heat loss.
How do inversions and isothermal layers affect the lapse rate?
Inversions cause the effect to reverse and the air actually heats up as it rises. Isothermal layers are where the temperature remains the same in that layer and doesn’t lapse.
Define latent heat.
The heat required to cause a change of state, the actual temperature does not change during the change of state as the energy is used to actually change the state.
How does the temperature of the air affect its humidity/saturation? What is the point at which a piece of air that is cooling becomes fully saturated?
Warmer air holds more moisture so as a parcel of air warms its ability to retain water increases so it is less saturated. The dew point.
What is condensation?
When the parcel of air passes its dew point when cooling and the water vapour in it returns to liquid form.
What four ways may a parcel of air be cooled past its dew point?
Conduction Evaporation Mixing Adiabatic cooling.
Define adiabatic
The change in temperature of a parcel of air as its density changes, normally cooling as the air expands when rising.
What are the adiabatic lapse rates for dry and saturated air?
SALR= 1.5 degrees/ 1000ft DALR= 3.0 degrees/1000ft
What is the ELR?
The environmental adiabatic lapse rate that exists within the atmosphere at a particular time. (ISA global average is 1.98 degrees per 1000ft)
Define stable and unstable air.
Stable air is air that doesn’t rise and unstable rises.
What is absolute stability?
When the ELR is less than both the DALR and SALR creating stable air.
What is conditional instability?
When the ELR lies between the SALR and DALR so dry air will be stable as it is cooler than the environment but saturated air will rise as it would be warmer.
What is absolute instability?
When the ELR is greater than both the SALR and DALR meaning that both dry and saturated air will be unstable.
What is the vertical distance associated with 1hPa change in pressure?
30ft
How many inches of mercury are there in 1013.25hPa?
29.92
What does an altimeter set to QNH read?
The altitude, i.e. Vertical distance between you and MSL
What does an altimeter set to QFE read?
The height above a specified datum, normally aerodrome elevation.
What is the SPS and what does it give you?
The standard pressure setting of 1013.25hPa and it gives you your vertical position expressed as a flight level.
What is a RPS?
A regional pressure setting, same as QNH but covers a region. Used by aircraft below 3000ft outside CAS. Lowest forecast QNH in the region.
Define transition altitude.
The altitude at or below which the vertical position of an aircraft is controlled by reference to altitude.
Define transition level.
The lowest flight level available for use above the transition altitude.
Define transition layer.
The airspace between the transition altitude and transition level.
What is the fundamental process that causes clouds to form?
Condensation, caused by the air being cooled past its dew point either by; Adiabatic cooling through air rising Heat loss by conduction Temperature drop as a result of mixing (e.g. Turbulence)
What processes can cause air to rise and therefore cool?
Convection currents due to localised surface heating Frontal activity between adjacent air masses of differing temps (aka mass ascent) Orographic ascent cause by terrain Turbulent motion over undulating ground
Describe the convection current method of cloud formation.
Insolation causes the earth to heat up and this energy is then reradiated out into the air causing the air closest to the ground to warm first. This air then rises until it hits its dew point at which point it condenses into cloud. The extent of the cloud is dependent on the humidity and the strength of the rising currents.
Describe the frontal activity method of cloud formation.
When a mass of air of one temperature encounters one at a different temperature the warmer air will be forced upwards and cooled adiabatically until it hits the dew point and condenses.
What is orographic ascent?
An air mass forced to rise by the terrain rising, this causes the air to cool adiabatically until its dew point is reached. If the air is stable the cloud will only form in the vicinity of the hill or mountain as the air will warm again on its descent down the other side.
Describe turbulent mixing with regards to cloud formation.
Air mass moving quickly over undulating ground has its lower levels of air retarded by friction with the ground or obstacles causing turbulence which can form low clouds. Not normally much vertical growth.
What are the four cloud families?
High level- above 20,000ft Medium level- above 6,500ft Low level- below 6,500ft Clouds of vertical extent- those which extend over multiple levels.
What are the types of clouds?
Cirrus-wispy Cumulus- fluffy sheep Alto- medium Stratus- layers Nimbus- rain bearing
Define cloud base.
The height of the base of any cloud above the aerodrome elevation.
Define cloud ceiling.
The vertical distance from the elevation of the aerodrome to the lowest part of any cloud visible from the aerodrome which obscures more than one half of the sky.
What instruments are used to measure cloud base/ceiling and how do they work?
Ceilometer. Fires lasers up into the sky and measures the reflection.
What is used to assess cloud cover and what are the categories?
Oktas (eights of the sky) 0= NSC (no significant cloud) 1-2= FEW 3-4= SCT (scattered) 5-7= BKN (broken) 8= OVC (overcast)
What is precipitation and what are its dimensions?
Any moisture that is released from the atmosphere in either liquid or solid form. Water droplets- 0.02mm Drizzle- 0.2mm Heavy rain- max 5.5mm Hail- can reach larger diameters
Why is precipitation an important factor?
Supercooled water droplets- icing Wet surfaces- braking action Visibility- VFR/IFR Engines- flooding
What do you call water droplets that are below 0 degrees but have not frozen?
Supercooled
What do supercooled droplets need to freeze?
A freezing nuclei e.g. Particulates, aircraft skins, or already frozen droplets.
What are the types of precipitation?
Snow Sleet Drizzle Rain Hail
What are the four ways in which clouds can form?
Conventional Turbulence Orographic Frontal
Detail the different levels of intensity and durations of rainfall.
Intensity- light, moderate and heavy Duration- short, intermittent, or continuous
What types of precipitation are associated with stratus or stratocumulus clouds?
Drizzle, freezing drizzle or snow grains
What types of precipitation are associated with thick Altostratus and nimbostratus clouds?
Continuous rain or snow.
What types of precipitation are associated with thick Altostratus and stratocumulus clouds?
Intermittent rain or snow
What types of clouds can produce rain or snow showers?
Altocumulus, large cumulus and cumulonimbus
What types of cloud produce hard or soft hail?
Cumulonimbus
What three conditions must be present for a thunderstorm to develop?
Moist air Unstable air Trigger